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Middle atmosphere dynamics. By D. G . Andrews, J. R. Holton and C. B. Leovy. Academic Press, San Diego, 1987. Pp. xi + 489. US$34.95.

Concern about the effects of man-made pollutants on the possibly fragile ozone layer in the stratosphere has stimulated a great deal of research on the photochemistry and dynamics of the atmosphere above the tropopause. Recently, scientists have been galvanized by the discovery that the amount of ozone over Antarctica declines precipitously in spring-leading to an ‘ozone hole’-and that there might even be a hint of a global reduction. Such unexpected findings exposed serious gaps in our knowledge of basic physical, chemical and dynamical processes, and led to animated debates about the underlying cause or causes. Some researchers suggested that the hole’s origin was meteorological; others that it was chemical (albeit involving anomalous chemistry). After some early skirmishes (when the grand prize of scientific priority was at stake), scientists generally accepted that the weight of evidence favoured a chemical explanation, and order was finally restored when some sages proposed that both chemistry and meteorology were relevant to the formation and subsequent evolution of the hole. A lesson to be drawn is that interdisciplinary expertise is needed to make headway with some of the pressing questions in middle atmosphere research.

A close study of Middle atmosphere dynamics by Andrews, Holton and Leovy (AHL) would be an excellent way to start to acquire not only a good understanding of the dynamical meteorology of the stratosphere and mesosphere (with which the book is mainly concerned), but also of radiative transfer in the region and, at a more elementary level, of the photochemistry influencing ozone amounts. The authors have produced an authoritative, lucid monograph which gives a well-balanced presentation of observations and theoretical ideas. A useful feature for beginning students is the annotated compendium of key research papers given at the end of each chapter. The book will surely appeal to a wide audience.

The reader’s confidence, gained by an easy-to-read introductory chapter, is put sternly to the test by the one that follows, on radiative processes and remote sounding. The density of information is high, but at least there is the consolation of knowing that one has to master only about ninety pages of text to give a reasonable impersonation of a radiation expert.

A somewhat more leisurely pace is adopted in the main body of the book dealing with middle atmosphere dynamics. Nonetheless, most benefit is to be gained by an active reading of the text-filling in the steps that the authors have omitted for brevity in writing down their equations.

After the basic dynamical equations are presented, the linear approximation is made to analyse the types of wave motion encountered in the middle atmosphere: tides, planetary waves, gravity waves and equatorial waves. Attention is then directed to the annual evolution of the circulation at extra-tropical latitudes, the discussion being based on measurements by satellite-borne radiometers. Recent theoretical and observational work has indicated that a nonlinear theory is needed to analyse the large-scale circulation of the middle atmosphere where planetary waves ‘break’. The authors give an elegant sketch of the kind of Row expected in a region of wave breaking and of effects on wave structure elsewhere.

One of the most spectacular phenomena encountered in the middle atmosphere is the stratospheric sudden warming, a sporadic event in winter during which the westerly Row is broken down and temperatures rise dramatically. Sudden warmings have been an enigma since their discovery more than thirty years ago. Although it is now accepted that they have their origin in large-scale disturbances in the troposphere, a full understanding is lacking-a point, I feel, not adequately conveyed by the book, in part because attention is restricted to one well observed and studied example.

Next, the authors consider the role of dynamical and radiative processes in controlling the zonally averaged circulation at extra-tropical latitudes. The discussion is illuminated by reference to a hierarchy of models of differing complexity. The importance for the middle atmosphere of upward propagating planetary and gravity waves from the troposphere is thereby clearly exposed.

Phenomena occur at equatorial latitudes in the middle atmosphere that are just as fascinating as sudden warmings in the extra-tropics. The quasi-biennial oscillation (QBO) is a long-term oscillation in equatorial winds, and is peculiar in that its period-averaging 27 months-is not an harmonic of the solar-driven annual cycle. An outstanding success of dynamical meteorology was the development of a convincing theory of the QBO. AHL give a clear outline of the theory and of a beautiful mechanical experiment that confirmed its main elements. The equatorial semi-annual oscillation receives an equally limpid treatment.

A pivotal chapter in the book is the one dealing with tracer transport. The circulation in the middle atmosphere transports trace species into different photochemical environments and thus may influence their concentrations. Consequently, models designed to give reliable predictions of the chemical impacts of pollutants should include some representation of transport. How best to do this in simplified (one- and two-dimensional) models has been a thorny problem for many years. In particular, the seemingly complicated picture of transport that emerged in a so-called Eulerian-mean framework (with mean circulations having the annoying habit of counteracting the effects of eddies) was difficult to reconcile with the more compelling picture afforded by a Lagrangian view. Fortunately, clarification has come in recent years, and with it the hope that the task of parametrizing transport in simplified models is not utterly hopeless. Summarizing a number of recent research papers, AHL show that, at least for small amplitude disturbances, transport equations may be cast in a form with desirable properties. The approach has didactic value, though success in practical applications is not guaranteed.

A short chapter on the ozone layer sketches the main features of the geographical distribution of ozone and of its temporal variability. An introduction to ozone photochemistry is then given. Little prior knowledge

422 REVIEWS

is required to follow the discussion, and dynamical meteorologists might at least be tempted to peruse weightier tomes on the subject, such as the recent book by Brasseur and Solomon.

As the amount of data from satellite-borne instruments has increased, the feeling has grown that sophisticated, high-resolution general circulation models are a vital adjunct of simplified (mechanistic) models in achieving a detailed understanding of the dynamical, physical and chemical processes at work in the middle atmosphere. Several such models have been employed by groups in different countries, and further development is likely to continue apace. AHL survey progress so far and point out some successes and failures of the modelling effort.

The book closes with a chapter on the interactions between the middle atmosphere and the lower atmosphere, comprising radiative and dynamical links. While acknowledging the strong upward effect of the troposphere on the middle atmosphere, the authors manage to cite a few mechanisms by which the middle atmosphere may exert a downward influence on the troposphere. N o doubt their words will be oft-quoted in grant applications by fund-starved middle atmosphere researchers.

ALAN O’NEILL

Tracers in fhe ocean. Edited by H. Charnock, J. E. Lovelock, P. S. Liss and M. Whitfield. .Royal Society, 1988. Pp. 236. f45.

The gut feeling of many, nay most, physical oceanographers is that chemical tracers as they are moved around the oceans by the flow carry with them information about thedynamics of the flow itself. Indeed, the two tracers temperature and salinity have been the mainstay of physical oceanography for many decades in defining water masses and their origins as well as being used in calculating geostrophic currents. Improvements in analytical chemistry have increased the suite of useable tracers. Interest in the use of tracers heightened when it was discovered that tritium from atmospheric bomb tests and later that freons have a clear signature in the ocean. These man-introduced tracers not only give information from their transient behaviour but also carry ‘clocks’ giving estimates of the age of the water.

This book is a collection of papers presented at a Royal Society discussion meeting held in May 1987. The meeting was timely in that, after the euphoria of the realization that we may well have a tool that ‘provides information about its (the ocean) circulation that cannot be obtained in any other way’, thought needs to be given to just how useful and useable the new information is, all the more so because of the forthcoming global hydrographic survey under the World Ocean Circulation Experiment. The papers in this book go some way in doing this, by presenting a number of diverse applications of tracer data.

The two opening papers deal with carbon dioxide (Sarmiento ef 01. and Crane), topical because of the ‘greenhouse effect’ and the need to understand the role of the ocean in determining atmospheric levels. Both papers present box models of the ocean and highlight the need to include the effects of ventilation of deep ocean layers but also show the strong dependence of results on shaky model parameters. Sarmiento ef al. go on to show results from a dynamical 3D model and to infer conclusions about the biogeochemistry of the ocean. I must say, I shared Prof. Wunsch’s scepticism, as given in the published discussion, about the value of the results from a model that has been shown to have ‘serious physical shortcomings’.

Transient tracers are put to good use by Jenkins and by Roether and Fuchs. Jenkins shows maps of tritium and the ‘tritium-helium-3’ age on isopycnal surfaces in the N Atlantic derived from data from the Transient Tracers in the Ocean expedition. The maps show clearly the majestic circulation at depth and the pathways of ventilation of these surfaces. One receives a cautionary tale from Roether and Fuchs who find an almost 180 degree discrepancy between the direction of the flow derived from tritium data and that from geostrophic and beta-spiral methods (but then see Bigg and Killworth!)

The air/sea exchange of gases is discussed by Thomas eta!. and by Liss. Liss provides estimates of transfer velocities using radioactive tracers and deliberate releases of stable tracers. Cause for concern is given in that the results contradict those using eddy correlation methods.

The perseverance of analytical chemists in forever reducing the detectable limits of substances in sea water has provided new sets of the more exotic tracers (Elderfield and Burton and Statham). Some datasets, however, such as those of trace metals, are still rather scant and will require many more days of ship-time before they can be used to their full potential.

The detection limit of some substances is so amazingly small that deliberate releases of these substances can be contemplated to study dispersion processes in the ocean (Watson and Ledwell). Feasibility studies have provided very promising results.

Extracting dynamical information from the tracer fields has been labelled the ‘inverse problem’. Many of the papers in this book address this problem. The results so far have been inconclusive. Professor Wunsch, one of the protagonists of the method, has set out to provide a theoretical framework in which to gauge the information content of tracer fields. In his paper to the meeting he concentrated on the transient tracers tritium and helium-3 and concludes that they place far fewer constraints on the flow field than do the density field and steady state tracers. His views are controversial and the debate rages on. Perhaps both sides are not eclectic enough.

The collection of papers gives a good perspective of the ‘state-of-the-art’ and by being brought together, the whole should be judged greater than its parts. There are two shortcomings of the book. Firstly, freons.